Power electronics are devices that are used in the distribution and collection of energy. These devices must be able to withstand large reverse biases, as well as significant forward currents for high power applications. Common applications for power semiconductor devices are AC-DC and DC-DC converters found in numerous consumer applications, including televisions, computers, and battery chargers. On the industrial scale, motor controls require devices suitable for their circuitry. Additionally, the push for highly efficient devices with minimal losses to heat and switching times is now important from both an economic as well as an ecological point of view.
One challenge in the design of power electronics, particularly for RF applications, is that many devices and materials are typically better suited only for high power or high speed applications, but not both. Current silicon-based technologies are reaching the performance levels indicated by several theoretical figures of merit in high-power and high-speed applications, requiring new material systems to be investigated. Gallium nitride (GaN) is an ideal candidate to meet both of these needs due to its wide bandgap, high critical breakdown field, and high mobility. Use of GaN in power electronics, however, brings additional challenges in the design of normally-off devices. Due to the inherent polarization field in hexagonal phase GaN, a conductive two-dimensional electron gas (2DEG) is formed under zero bias conditions. Normally-off devices, however, are sought for high-power applications for reasons of safety, e.g., circuit protection, and power savings, for example.
Several methods currently exist to create a normally-off GaN device, including fluorine implantation below the gate, recessed gate approaches, and p-GaN gates. However, normally-off devices enabled by these approaches are yet to mature. Recently, inverted GaN or AlGaInN hetero-structures and N-polar structures were explored for p-channel field-effect transistors (FETs) without success.